Annulation

About: Annulation is a research topic. Over the lifetime, 10152 publications have been published within this topic receiving 189701 citations.

Azomethine ylide cycloaddition/reductive heterocyclization approach to oxindole alkaloids: asymmetric synthesis of (-)-horsfiline.

Abstract: The intermolecular [3 + 2] annulation of azomethine ylides with 2(2-nitrophenyl)acrylate dienophiles followed by reductive heterocyclization affords the spiro(indole-pyrrolidine) ring system. Hence, this enable us to accomplish a concise and highly enantioselective synthesis of (-)-horsfiline 1, based on chiral auxiliary-directed pi-face discrimination in the 1,3-dipolar cycloaddition of (1S,2R)-2-phenyl-1-cyclohexyl ester 4f with N-methylazomethine ylide.

Asymmetric cascade annulation based on enantioselective oxa-Diels-Alder cycloaddition of in situ generated isochromenyliums by cooperative binary catalysis of Pd(OAc)2 and (S)-Trip.

Abstract: An asymmetric cascade annulation between 2-hydroxystyrenes and 2-alkynylbenaldehyes or 1-(2-alkynylphenyl)ketones has been established with good to excellent enantioselectivities (up to >99.5% ee), on the basis of an enantioselective oxa-Diels–Alder cycloaddition of in situ generated metallo-isochromenylium intermediates, by cooperative binary catalysis of Pd(OAc)2 and (S)-Trip. The developed methodology is workable for a broad spectrum of substrates and shows great efficiency in establishing dense multiple chiral centers including quaternary carbons of variable bridged ring systems. The mechanism study suggests that (S)-Trip plays multiple roles in assembling the reactants and controlling the stereoselectivity.

Intermolecular oxidative annulation of 2-aminoanthracenes to diazaacenes and aza[7]helicenes.

Abstract: Aniline is oxidized into so-called “aniline black”, well-known black pigments that are a mixture of oligomeric materials containing C N double bonds and fused structures. Since such an oxidative fusion reaction of aniline derivatives is usually uncontrollable and provides many isomers and oligomers, it has rarely been recognized as a synthetically useful transformation. However, the selective oxidation of aromatic amines should provide a powerful protocol to combine two aromatic units into one extended p system if the substrates and reaction conditions are carefully designed. Recently, oligoacenes containing pyrazine units have attracted much attention as promising compounds for electron-transporting materials because of their resistance to oxidation relative to the parent oligoacenes. However, the synthetic strategy for this type of azaacenes has mainly been limited to the classic condensation of ortho-quinone derivatives with aromatic ortho-diamines, both of which are often unstable under air. Here we disclose the selective oxidative fusion of 2-aminoanthracenes mediated by 2,3-dichloro-5,6dicyano-1,4-benzoquinone (DDQ) to provide pyrazine-fused bisanthracenes in high yields. Our approach, which makes use of the more-stable monoamino substrates, would be useful for the construction of various pyrazine-fused oligoacenes. Furthermore, we also found that pyrrole-fused bisanthracenes could be formed under slightly modified conditions. The pyrrole-fused dimer can be regarded as an aza[7]helicene with a stable helical conformation that exhibits circular dichroism (CD) and circularly polarized luminescence (CPL) properties. We used 2-aminoanthracene 1a, the two triisopropylsilylethynyl groups of which enhance the solubility of the starting material and products. We found that the oxidation of 1a with DDQ in CHCl3 (Scheme 1) afforded a pyrazine-fused dimer 2a in 43 % yield as a single regioisomer (Table 1, entry 1). The H NMR spectrum of 2 a was indicative of a C2h symmetrical structure, thus indicating a zig-zag-type structure fused by a pyrazine linkage. Furthermore, another product 3a was obtained as a more polar material in 48 % yield. The H NMR spectrum showed 3a was unsymmetrical, with one NH proton signal detected at d = 9.12 ppm. The parent mass ion peaks of 3a at m/z = 1087.6716 (calcd for (C72H97NSi4) + = 1087.6693) suggested a loss of one nitrogen atom from 2a. On the basis of these data, we assigned 3a as a pyrrole-fused dimer. Finally,

Diastereoselective [3+2] Annulation of Aromatic/Vinylic Amides with Bicyclic Alkenes through Cobalt-Catalyzed C−H Activation and Intramolecular Nucleophilic Addition

Abstract: A highly diastereoselective method for the synthesis of dihydroepoxybenzofluorenone derivatives from aromatic/vinylic amides and bicyclic alkenes is described. This new transformation proceeds through cobalt-catalyzed C-H activation and intramolecular nucleophilic addition to the amide functional group. Transition-metal-catalyzed C-H activation reactions of secondary amides with alkenes usually lead to [4+2] or [4+1] annulation; to the best of our knowledge, this is the first time that a [3+2] cycloaddition is described in this context. The reaction proceeds under mild conditions and tolerates a wide range of functional groups. Mechanistic studies imply that the C-H bond cleavage may be the rate-limiting step.

Double Isocyanide Cyclization: A Synthetic Strategy for Two-Carbon-Tethered Pyrrole/Oxazole Pairs

Abstract: A new strategy for the construction of the compounds with two different heterocycles, linked by a C(2)-tether via a domino process involving [5 + 1] annulation, ring-opening, and subsequent double isocyanide cyclization, from the reaction of ethyl isocyanoacetate with divinyl ketones (DVKs) has been developed. The chemoselective fragmentation of the cyclohexanone intermediate is the key for the formation of not only the C(2)-tether but also the two different heterocycles.